In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of radio frequency (RF) current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by RF current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
Although electrodeless HID lamps generally provide good color rendition and high efficacy in accordance with the standards of general purpose illumination, if unshielded, such lamps typically produce electromagnetic interference (EMI) which adversely affects, for example, radio and television reception. Therefore, it is desirable to provide electrodeless HID lamps exhibiting reduced EMI without appreciable reduction in visible light output, thus making such lamps practicable for widespread general illumination applications.